The lead-acid battery has been a successful secondary battery system for over a century [1]. The advantages of these batteries are their low cost, stable voltage profile, high reliability, and safety. The main disadvantages of such flooded type configuration are a low specific energy and, subsequently, the poor utilization of the positive active-material (PAM). Both the cycle life and the capacity of these batteries are limited due to the properties of the active material in the positive plate. Compared to the negative plate, the positive plate has low performance in deep discharge (high DOD), and therefore much research has been conducted to improve the performance of lead-acid batteries [2].
The low utilization of PAM stems from the associated sulfation and crumbling of the active material. The crumbling of the active material originates from the significant difference between the densities of the PbO2 and PbSO4, leading to expansion of the active material that occurs during the discharge process [3]. During discharging, non-conductive crystals of PbSO4 are formed. When these electrically insulating crystals grow, as either plates or spatial crystals, they either prevent the lead ion oxidation back to the active PbO2 material or reach a stable crystal size (above 1-1.5 micron in diameter), which cannot be recharged via the common dissolution/precipitation mechanism.
Carbonaceous additives were studied mainly for negative active materials of valve-regulated lead acid (VRLA) cells. Amongst others, their contribution may be associated to the enhancement of the overall conductivity of the active material, facilitation of the formation of small isolated PbSO4 particles which are easy to dissolve, and the ability of carbon to act as an electro-osmotic pump that facilitate acid diffusion within the inner volume of the active material, especially at high rates of charge and discharge. Pavlov et al. [4] suggested a mechanism for the role of the carbonaceous additives in the negative active material. According to the proposed mechanism, the lead sulfate dissolves and diffuses to conductive sites (i.e. pure lead, extremely thin lead sulfate layer or the carbonaceous additives surface), in which it may be reduced into metallic lead due to its sufficient electrical conductivity. At later stages, due to the mismatch in the crystal lattice parameters, the reduced lead diffuses from the carbon surface, releasing the electro-active sites on the carbonaceous surface to be available for further reduction.
Among the various carbon allotropes, carbon nanotubes (CNTs) seem to be a prominent additive due to their outstanding features, including high mechanical properties, and excellent electric and thermal conductivities. Many of these properties are best exploited by incorporation of CNTs into composites [5]. The extremely high aspect ratio of the CNT (up to 106) turns the formation of dispersion into a challenge, as there is a need to overcome all of the local Van der Waals interactions which tend to hold the CNTs macro-scale bundles intact [6].
Chemical methods have been used to surface-functionalize CNTs to improve their chemical compatibility with a target medium (solvent or polymer solution/melt), to enhance wetting or adhesion characteristics and reduce their tendency to agglomerate [7].